Improving treatment processes through desludging settlement lagoons

Desludging settlement lagoons

Our client

Our client is the Coal Authority.

The location is a mine water treatment scheme in South Yorkshire.

The two settlement lagoons – with an 11,000m3 capacity – form part of the mine water treatment process.

The situation

Oren Environmental operates and maintains 76 mine water treatment schemes across the UK for the Coal Authority – as well as reed bed maintenance and large-scale desludging projects for various commercial organisations.

As industry leaders in our sector, the Coal Authority asked us to desludge two settlement lagoons at their Sheephouse Wood mine water treatment scheme, which was built on the former Hand Bank Colliery site, which closed in the 1930s.

The two settlement lagoons form part of the treatment process that removes iron oxide from the mine water to prevent it from polluting nearby watercourses and harming ecosystems.

Over time, settlement lagoons can get choked with vegetation. Additionally, heavy metals found in the mine water drop to the bottom of the settlement lagoon as part of the treatment design. These solids combine and, over time, reduce the capacity of the available treatment area. What’s more, with the same amount of water travelling at velocity through a smaller space within the settlement lagoon, solids get carried over to the neighbouring reed bed, which causes it to become blinded.

This is why settlement lagoons need to be regularly cleaned and maintained. The Coal Authority recognises this and appreciates that continual maintenance keeps treatment processes running optimally and prevents watercourse pollution.

Settlement lagoon vegetation prior to works

Our approach

We removed the top water from the two settlement lagoons and stored it in tankers so we could reuse it for jetting. This honours our commitment to sustainability by recycling water whenever possible.

Large amounts of sludge were decanted from the settlement lagoons into a newly refurbished sludge drying bed. This dewatering technique removes water content from the sludge, making it easier to transport in fewer trips. This sustainable approach is also more cost-effective for our clients.

One of the settlement lagoons had significant quantities of vegetation that had to be excavated using suction to avoid further damage to the liner—again, keeping costs down for our client. Vegetation below the settlement pond had burst through the baseliner, which was patched and repaired.

Drained settlement lagoon
Damaged then repaired settlement lagoon liner

Obstacles we overcame

Sludge volume and thickness

The design of the settlement lagoon was intended to allow auto desludging via valve operation. However, due to the density of the sludge, it needed a little encouragement. We achieve this by reintroducing the surface water retained in the tankers and/or opening valves to allow a small amount of pumped mine water back into the lagoon to aid the ochre suspension, enabling the ochrous sludge to flow more freely.

Large volume of sludge

Maintaining compliance

Throughout the process, we monitored and took samples from the discharge points using onsite testing equipment and lab-supported processes to satisfy environmental consent to discharge requirements.

Our impact

As a result of our work, we’ve:

  • The Coal Authority now have two settlement lagoons at Sheephouse Wood free from excess sludge and vegetation, so they hold more water and retain it for longer – improving the treatment process
  • The sludge-drying bed we constructed and installed now dewaters sludge, making it easier and cheaper to transport and recycle
  • The sludge is reused as spread for farmland. This happens after it’s dried out and deemed stackable for transportation
  • The nearby watercourses will remain protected from mine water pollution in the future

Works were delivered on time and to budget, as per our client’s expectations.

Jetting using recycled water